The present invention relates to a device for determining the filling level of a filling material in a container.
The device has a signal-generating unit, which generates electromagnetic measuring signals, a transceiver unit, which emits the measuring signals via an antenna in the direction of the surface of the filling material and which receives the echo signals reflected at the surface of the filling material, the antenna comprising a waveguide which widens in the direction of emission into a cavity of a defined shape, and an evaluation unit which determines the filling level in the container from the propagation time of the measuring signals.
Devices of the above-named type preferably operate in the microwave region and are generally known under the name of microwave filling level measuring instruments. However, it is also possible in principle to operate the device according to the present invention with the aid of ultrasonic measuring signals.
The formation of sediments, what is termed deposition formation, in the antenna region is critical for all measuring instruments which determine the filling level of a filling material in a container via the propagation time of measuring signals. If a deposit, that is to say condensate and/or moisture as well as contamination of any sort, settles in the interior of the antenna, this has a direct influence on the propagation and the reflection behavior of the measuring signals: interference signals occur which are superimposed on the actual measuring signals and can become so strong as finally to render reliable filling level measurement impossible.
It has become known from the prior art to protect an antenna against deposit formation by means of what is termed a radome. A radome is a thin, film-like protective layer which is attached in the region of the outer edge of the antenna and prevents condensate and/or moisture from being able to penetrate into the interior of the antenna. The disadvantage of the radome is that because of its fragile configuration it has little resistance to high pressures. Its use in the field of process metrology is therefore relatively highly restricted.
It is an object of the present invention to provide a stable device for filling level measurement in the case of which deposit formation is excluded in particularly critical regions of the antenna such as for example the interior of the antenna.
The object is achieved by virtue of the fact that at least one dielectric filling material is provided which at least partially fills up the waveguide, and which virtually completely fills up the widened cavity of the antenna. The use of an insert made from a dielectric material renders it possiblexe2x80x94if desiredxe2x80x94simultaneously to eliminate a plurality of problems which can occur with antennas of the prior art:
The antenna has a very stable structure.
The antenna is effectively protected against deposit formation.
A high resistance to pressure is achieved for the antenna by the use of the dielectric material.
The chosen design of the dielectric material introduced into the interior of the antenna assists the focusing of the electromagnetic measuring signals.
Moreover, the widened cavity preferably has the shape of a funnel. Correspondingly fashioned antennas are usually termed horn antennas. In a horn antenna, the inner surface can either be smooth, or it has a certain structure. Horn antennas with a structured inner surface are generally known under the designation of xe2x80x9ccorrugated horn antennasxe2x80x9d. The purpose of the structured inner surface of the antenna is to impress on the measuring signals a phase shift dimensioned such that a substantially plane wavefront is emitted at the antenna output.
The widened cavity of the antenna is, however, not limited to the shape of a funnel, as is customary with horn antennas. For example, the widened cavity can also have the shape of a parabolic mirror, or else a cylindrical shape.
Basically, the filling material can be any desired dielectric material. Teflon, sapphire, ceramic or quartz glass, for example, will be applied as filling material.
In accordance with a preferred refinement of the device according to the invention, the outer surface of the filling material is shaped as a lens. The outer surface of the filling material can thus be of convex configuration. Likewise, the outer surface of the filling material can also have the shape of a Fresnel lens.
In order to avoid damage to the filling material during transportation, in the course of installation or dismounting or while storing the antenna, according to an advantageous development of the device according to the invention the highest elevation of the outer surface of the filling material is arranged under the outer edge of the widened cavity of the antenna.
In accordance with an advantageous refinement of the device according to the invention, an additional protective layer is provided which is arranged upstream of the dielectric filling material in the direction of emission. This additional protective layer is preferably used whenever the outer surface of the filling material is structuredxe2x80x94in which case it must be considered that a structured outer surface accommodates deposit formation. This protective layer can either be of plane configuration, but it can also have the shape, for example, of a cone turned inside or outside.
The frequency of the high-frequency electromagnetic measuring signals is preferably higher than 30 GHz. The antennas can be of relatively small configuration if the frequency is of this order of magnitude. Deposit formation is highly problematical precisely in the case of small antennas, since the internal dimensions of the waveguide must be small by comparison with the size of drops of condensate or particles of dirt which make up the deposit. This means that frequently as much as a single drop or single particle of dirt suffices to block the waveguide and cause very strong interfering reflections. If the antenna cavity is filled up according to the invention with a dielectric, the waveguide and all the antenna regions with small internal dimensions are protected effectively against condensate and other forms of contamination. Deposit formation is less critical on the relatively large outer surface of the dielectric, since individual drops or particles do not cover the entire diameter and can drop off again more easily.
As already mentioned at an earlier juncture, it is possible in addition to achieve a focusing effect through the surface shape and surface structure of the filling material: the region in which the measuring signals impinge on the surface of the filling material is precisely defined, as a result of which interference signals, that is to say measuring signals which are reflected not at the surface of the filling material, but at any built-in components in the container or at the container wall, become less likely.
In accordance with a preferred development of the device according to the invention, the transceiver unit guides the electromagnetic measuring signals through the rear wall or through the side wall of the waveguide into the antenna or out of the antenna.
A solution which is particularly advantageous because it is cost-effective provides that the dielectric filling material fills up only a subregion of the waveguide, said subregion adjoining the widened cavity of the antenna. As an example, an antenna may be named here in the case of which the length of the waveguide turns out to be relatively large for reasons of temperature coupling between filling level sensor and process/process medium. If the filling material fills up the cavity of the antenna only partially, it is particularly favorable when the end region of the dielectric filling material, which extends into the waveguide, is shaped so as to optimize the coupling of electromagnetic measuring signals at the air/filling material transition.